C++: Clarify which taint tracking libraries should be used somewhat.

Author: geoffw0

cpp/ql/src/semmle/code/cpp/security/TaintTracking.qll

@@ -1,5 +1,7 @@
 /*
  * Support for tracking tainted data through the program.
+ *
+ * Prefer to use `semmle.code.cpp.dataflow.TaintTracking` when designing new queries.
  */
 
 import semmle.code.cpp.ir.dataflow.DefaultTaintTracking

cpp/ql/src/semmle/code/cpp/security/TaintTrackingImpl.qll

@@ -1,4 +1,8 @@
 /**
+ * DEPRECATED: we now use `semmle.code.cpp.ir.dataflow.DefaultTaintTracking`,
+ * which is based on the IR but designed to behave similarly to this old
+ * libarary.
+ *
  * Provides the implementation of `semmle.code.cpp.security.TaintTracking`. Do
  * not import this file directly.
  */